Carbon isotopic distribution of methane in deep-sea hydrothermal plume, Myojin Knoll Caldera, Izu-Bonin arc: implications for microbial methane oxidation in the oceans and applications to heat flux estimation

Abstract

The concentration and stable carbon isotopic composition (δ 13C) of methane have been measured for both seafloor hydrothermal venting fluids and effluent plume waters supplied from the vents at the Myojin Knoll Caldera, Izu-Bonin arc, in the western North Pacific. The hydrothermal end-member concentrations and δ 13C of methane show near-homogeneity among vents: 41.2 (μmol/kg) and −16.3 ± 0.8 (‰PDB), respectively, while those in the effluent plume are stratified in the caldera and vertically exhibit a large variation (2.1–11 (nmol/kg) and −29.0 to −11.3 (‰PDB), respectively). Comparison of concentration and δ 13C data between vent fluids and plume waters suggest that the methane is not conserved but microbially oxidized along with the plume. Gradual decrease of diffusive methane flux in proportion to distance from the vent fields supports the occurrence of significant microbial oxidation within the plume. Assuming steady state emission of methane from the vents, (1) kinetic isotope effect due to the microbial oxidation (k 12/k 13); (2) methane flux from the vents (F vents); (3) heat flux from the vents (Q); and (4) average turnover time of hydrothermal methane (T all) are estimated to be k 12/k 13 = 1.005 ± 0.001, F vents = 90–340 (mol/d), Q = 30–110 (MW), and T all = 60–240 (d), respectively. The estimated turnover times, however, are not uniform within the water column. Around the vents, a turnover time of less than 50 d for methane is one of the shortest values in the pelagic ocean, while those at the distant points correspond to values more typical for deep ocean water.